Chalcogenide glasses belong to a class of materials that exhibit a number of interesting electrical and optical properties. Amorphous selenium (a-Se), for instance, is a particularly prominent type of chalcogenide glass. It has photonconductive properties and has been used in a variety of devices including for example, photocopiers, medical imaging systems and high-definition television broadcasting cameras.
Photoconductors are materials that are sensitive to visible light and other types of electromagnetic radiation (i.e. x-rays, ultraviolet radiation, infrared radiation etc.). When photons strike a photoconductor, they produce, through the process of ionization, two types of electric charge known as electrons and holes. Subjecting the photoconductor to an electric field while photons strike the photoconductor results in the holes and electrons flowing freely through the photoconductive material in opposite directions along the orientation of the electric field. Measuring the number of holes or electrons generated in the photoconductive material yields a resulting electric signal that has a magnitude directly related to the number of photons striking a particular region of the photoconductive material.
Amorphous photoconductors have an important advantage over conventional crystalline photoconductors (e.g. silicon) in that they can be easily manufactured over large areas by employing thermal evaporation, sputtering and vapour deposition processes.
In most cases, amorphous chalcogenide photoconductors (ACPs) are biased at relatively low electric fields (e.g. 10 V/μm for a-Se). At much higher electric fields (e.g. 80 V/μm and larger for a-Se), multiplication of charge carriers occurs. This property of multiplication of charge carriers, known as avalanche multiplication, is an important feature of certain ACPs such as a-Se, and has been utilized in a television camera that is more sensitive than the human eye as described in the publication entitled “Ultrahigh-sensitivity HDTV new Super-HARP camera” authored by K. Miyakawa et al. and published in Proc. SPIE 5677, 26-34, 2005.
The key limitation to utilizing the avalanche properties of ACPs in solid-state devices for very high sensitivity applications such as medical imaging has to do with the high electric fields required to precipitate hole and electron avalanche. Every so often when an ACP is biased with an electric field, electrical discharges occur in the biased ACP. At the high electric fields required for avalanche multiplication, these electrical discharges produce an electric current that rises uncontrollably. This effect, referred to as incipient breakdown, has the unfortunate consequence of irreversibly altering the structure of the photoconductive material of the ACP. Specifically, the high electric current at incipient breakdown heats up the ACP to the point where the photoconductive material of the ACP crystallizes.
To-date, avalanche multiplication in ACPs has only been used successfully in vacuum tube devices in which the surface of the ACP is scanned by an electron beam as described in the above-identified Miyakawa et al. publication. These vacuum tube devices, which are based on a-Se, derive their gain solely from the multiplication of a single charge carrier type (holes), as they are unable to support the higher electric fields at which the second charge carrier type (electrons) avalanches. The highest gain achieved to-date in these vacuum tube devices is approximately 1000.
There are two primary avalanche regions of operation in most solid-state avalanche photoconductors, namely the linear region and the Geiger region. In the linear region, only one type of charge carrier (holes) avalanches and the multiplication gain is simply proportional to the initial number of photogenerated charges. In the Geiger region, both charge carriers (i.e. holes and electrons) avalanche. When a charge carrier undergoes impact ionization, which is the underlying process behind avalanche multiplication, the charge carrier produces two new carriers of opposite charge. The newly-generated hole and electron are drawn towards opposite sides of the ACP, in turn producing more impact ionization events. This chain reaction can rapidly grow without bounds and eventually causes the electric field inside the ACP to collapse. In this case, the mode of operation of the ACP is highly nonlinear since any number of initially photogenerated carriers produces the same effect.
Known prior art discloses the use of amorphous selenium (a-Se) in high sensitivity solid-state photodetectors and imagers. For example, U.S. Pat. No. 5,818,052 to Elabd discloses a broad band solid state image sensor responsive up to and beyond the 1 μm cutoff wavelength, for use in a camera capable of imaging under very low light level conditions with good modulation transfer efficiency resulting in high resolution. The image sensor, which achieves high sensitivity at low light level, comprises a photoconductor with high avalanche detection gain, a silicon detector with very high gain pixel level amplifiers and noise suppression circuits. In particular, a high gain avalanche rushing photoconductor (HARP) sensor device is connected to an amplified metal-oxide silicon (AMOS) device and a low-noise read-out device. The high sensitivity image sensor device is fabricated by depositing an amorphous selenium photoconductive layer on top of a silicon junction diode or a palladium silicide (Pd2Si) Schottky barrier diode that is connected to the AMOS pixel amplifier circuits to form a two story AMOS device.
U.S. Pat. No. 5,892,222 to Elabd discloses a low light level detection and imaging device including a photon sensing and counting device for image detection that is capable of detecting/imaging low photon flux levels over a wide spectral range using either image tube or solid state readout. The photon sensing and counting device is composed of a detector stack having several photoconductive layers, at least one layer of the stack being an amorphous selenium layer that is capable of high gain avalanche multiplication. The detector stack further includes an amorphous silicon layer deposited on the amorphous selenium layer to absorb infrared (IR) and ultraviolet radiation to enhance responsivity in the red and near-IR region. The purpose of the amorphous selenium layer is to provide high responsivity in the blue region and also to provide avalanche gain or multiplication of the photo generated carriers in the amorphous silicon or selenium layers.
U.S. Patent Application Publication No. 2001/0020690 to Yasuda et al. discloses an image recording sheet and a solid-state image detector. The image recording sheet has a stimulable phosphor layer laminated on a base, and the solid-state image detector includes a photoconductive layer containing amorphous selenium as its main component and electrodes disposed on opposite sides of the photoconductive layer. An electric field is applied across the photoconductive layer so that avalanche amplification is obtained within the photoconductive layer. The image recording sheet is scanned under the electric field with stimulating light of wavelength of about 600 nm. Photostimulated luminescence light of wavelength of about 400 nm emitted from the fluorescent layer is incident on the photoconductive layer via an optical guide and a stimulating light cut filter. Electric charge generated within the photoconductive layer is detected by a current detecting circuit, whereby a radiation image signal is obtained.
International PCT Application No. WO 2005/103762 to Lee discloses a flat panel imager that generates an electronic x-ray image though direct conversion of x-ray energy to electrical charges in a selenium-based layer at first electrical fields of the order of 10 volts per micrometer established therein between upper and lower electrodes. At least one electrically conductive grid extends generally laterally at a level above but close to a lower surface of the selenium-based layer, and is biased to establish higher electrical fields in portions of the selenium-based layer below the grid in the order of at least 75 volts per micrometer thereby to promote an avalanche effect in the high-field portion of the selenium-based layer.
Although the above-references show solid-state photodetector/imaging devices employing amorphous selenium that take advantage of avalanche multiplication, to-date none of these solid-state photodetector/imaging devices have been commercialized, primarily because of the limitation associated with incipient breakdown at high electric fields (i.e. fields greater than 10 V/um in a-Se). Furthermore, these solid-state photodetector/imaging devices only take advantage of avalanche multiplication of a single type of charge carrier (holes). This is because the two types of charge carriers have different mobilities and, as such, for both charge carrier types (holes and electrons) to avalanche, a higher electric field (i.e. 110 V/um in a-Se) is required than for a single charge carrier to avalanche. As mentioned above, the need to apply a higher electric field further compounds the incipient breakdown problem since the amount of energy stored across the photoconductor grows quadratically as a function of the bias potential.
As will be appreciated, improvements in photodetector/imaging devices that take advantage of avalanche multiplication are desired. It is therefore an object of the present invention to provide a novel photodetector/imaging device with avalanche gain.